The present invention relates to a method of and an apparatus for determining the extent of the axial play between the stator and the rotor of an electric motor, especially of a small-size electric motor.
It is well known in the electric motor manufacturing industry, and especially in the branch thereof which deals with the manufacture of small-size electric motors such as those used in household appliances and the like, that the axial play between the shaft of the rotor and the axially effective bearings which are mounted on the stator of the electric motor is to be reduced or eliminated altogether in order to obtain a reliable and efficient operation of the electric motor after the latter has been put into operation. This is especially true of the aforementioned small-size electric motors which have rather simple constructions and which are to be relatively inexpensive so that the additional cost incurred in manufacturing the various components to exact narrow tolerances is to be avoided so long as the wider tolerances do not deleteriously influence the performance of the respective electric motor or can be eliminated upon assembly of the components.
Bearing this in mind, there has already been proposed an electric motor construction in which an axial play exists between the axial ends of the shaft of the rotor and two axial bearing members mounted on the stator and confining the shaft between themselves upon assembly of the rotor with the stator of the electric motor. The position of one of the bearing members relative to the stator in the axial direction, and thus the axial distance between the bearing members, is adjustable, for instance, in that the one bearing member is provided with external threads which mesh with corresponding internal threads of the stator, so that the turning of the one bearing member about its axis will result in axial displacement thereof.
In this conventional construction, it is necessary first to establish the extent of the axial play between the rotor or its shaft and the axial bearing members, before it is possible to properly position the displaceable bearing member such that the axial play is reduced to a minimum if not eliminated altogether. One known method suited for this purpose is to transfer the assembled electric motor to a play-adjusting station where the terminals of this electric motor are connected to a source of alternating current, so that the rotor commences its rotation. At this time, the presence of the axial play does not have any detrimental effect since the electric motor is being run under no-load conditions. An important objective to be achieved by this trail running of the electric motor is to let the bearings set and to permit the commutator system to run in. Depending on the degree of automation of the series production of such electric motor, impacts from pressurized-air stamping devices or from manually handled synthetic plastic material hammers are directed against the stator or housing of the electric motor, to aid in the setting of the bearings.
Once this is accomplished, the adjustable bearing member, which is constructed as a setting screw in this construction, is manually tightened to such a degree that an analog current measuring device which is interposed between the electric source and the terminals of the electric motor indicates a multiple of the idle run current consumed by the electric motor after the operation of the electric motor has been commenced. During this short-term overloading, the non-adjustable bearing member sets in the axial direction. Another operation accomplished as a result of this excessive tightening is a secure seating of the abutment members which are inserted into the shaft of the rotor at its two ends. When these abutment members are made as caps of synthetic plastic material which contact the respective bearing members, as they often are in small-size electric motors, the excessive tightening of the adjustable bearing member or setting screw also results in a desirable partial flattening thereof. Thereafter, the setting screw is loosened and then again tightened, but this time only until the analog current measuring device indicates that the electric motor consumes an amount of electric current which is only insignificantly higher than the idle current consumed by the electric motor when the axial play still existed. In this manner, the adjustment of the longitudinal or axial play is terminated, and the setting screw is affixed in its then assumed position.
A significant disadvantage of this conventional approach stems from the fact that the bearings, the commutator system, and the transmission which is present in the stator housing of this conventional small-size electric motor are not completely the same in terms of mechanical behavior from one motor of the series to another. The encountered differences in mechanical behavior and properties cause different consumptions of the electric current when the electric motors are being operated in the idle mode, which differences may then be reflected in an inaccurate adjustment of the axial play. In order to at least partially compensate for this disadvantageous effect, the person tightening the setting screw has to have a special experience and be very dexterous. This situation brings about certain problems especially when it is desired to increase the degree of automation of the manufacturing process.